Vehicles sold in North America are required to adsorb refueling, diurnal and running loss vapors into a carbon canister. When the canister is loaded with fuel vapor, the contents may be purged to engine intake using engine intake vacuum to draw fresh air though the canister, desorbing bound hydrocarbons. Strict regulations regulate the performance of evaporative emissions systems.
Hybrid vehicles, and other vehicles configured to operate with minimal or no intake vacuum may have limited opportunities to purge the fuel vapor canister. Even in standard engine vehicles, the fuel vapor canister may not be completely cleared of contents following a purge, as the airflow through the canister is not uniform. If the vehicle is parked in a hot or sunny location over a diurnal cycle, the retained hydrocarbons may desorb from the canister and result in bleed emissions.
Bleed emissions may be limited by adding a secondary “bleed” canister to capture desorbed hydrocarbons. However, this adds additional cost, weight, and packaging to the vehicle. Even with a bleed canister, a car parked outside for multiple diurnal cycles may experience bleed from the bleed canister itself. Further, in hybrid vehicles, a highly restrictive bleed canister may impede fuel tank depressurization prior to a refueling sequence.
The inventors herein have recognized the above issues, and have developed systems and methods to at least partially address them. In one example, a method for a fuel system, comprising: during a first condition, including an engine-off condition and a sealed fuel tank, directing fresh air into a fuel vapor canister responsive to detecting hydrocarbons in a canister vent line. By directing fresh air into the fuel vapor canister, fuel vapor stored within the canister may be redistributed away from the canister vent line. In this way bleed emissions from the fuel system may be reduced without the need for a dedicated bleed canister.
In another example, a fuel system, comprising: a fuel vapor canister; an air pump coupled between a vent port of the fuel vapor canister and atmosphere; and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: redistribute fuel vapor within the fuel vapor canister away from the vent port by activating the air pump. By redistributing the fuel vapor away from the vent port, bleed emissions may be reduced in vehicles parked in warm or sunny conditions for multiple diurnal cycles. In this way, hybrid vehicles and other vehicles which have limited opportunities to purge to intake during operation may meet stringent emissions standards.
In yet another example, a method for a vehicle, comprising: responsive to a vehicle-off event, setting a hydrocarbon breakthrough detection strategy; responsive to a hydrocarbon sensor output increasing above a threshold, opening a canister purge valve coupled within a canister purge line; coupling a canister vent port to the canister purge line; coupling a canister purge port to a vacuum pump; maintaining a fuel tank isolation valve closed; and activating the vacuum pump. By setting a hydrocarbon breakthrough detection strategy, battery power may be conserved while maintaining the ability of the hydrocarbon sensor to detect canister breakthrough. In this way, bleed emissions may be effectively countered based on the output of the hydrocarbon sensor.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.